1. Field of the Invention
The present invention relates to a hydrostatic stepless transmission including a hydraulic pump and a hydraulic motor, in which an angle/angles of a movable swash plate/plates of the hydraulic pump or/and motor is/are controlled.
2. Related Art
Conventionally, there is a well-known hydrostatic stepless transmission (hereinafter referred to as “HST”) including a hydraulic pump and a hydraulic motor, with a hydraulic servomechanism for controlling a tilt angle/angles of a movable swash plate/plates of the variable displacement hydraulic pump or/and motor. One conventional hydraulic servomechanism is an automotive type having an electromagnetic valve/valves connected to the hydraulic pump or the HST so as to automatically tilt the movable swash plate/plates in proportion to increase of the rotary speed of the hydraulic pump. Another conventional hydraulic servomechanism is a manual servomechanism for controlling the movable swash plate of the hydraulic pump by operating a speed control operation lever provided outside the HST.
As disclosed in JP 2004-11769 A, an HST of this conventional type is provided with a load controlling system such that a working vehicle is decelerated when load on an engine is equal to or exceeds a threshold value, and the working vehicle is accelerated when the load is equal to or lower than a threshold value.
With respect to the load controlling system with the HST, more specifically, the movable swash plate of the hydraulic pump or motor is tilted in the direction for decelerating the vehicle so as to prevent load on the engine from exceeding the threshold value, thereby preventing the engine from stalling. In this regard, with respect to an HST-controlled working vehicle, such as a tractor equipped with a backhoe and a loader, when a speed control operation device is operated for accelerating the vehicle at work, the output rotary speed of the HST is increased, however, the engine torque is reduced and the engine may stall in the worst case. Therefore, load on an engine is detected so that, when the engine is recognized to be overloaded, the HST speed control operation device is operated for decelerating the vehicle so as to prevent the engine from stalling.
With respect to this load controlling system with HST, to accurately control the tilt angle/angles of the movable swash plate/plates of the hydraulic pump or/and motor, it is noticed that variation of hydraulic pressure in a main fluid passage of the closed circuit of the HST connecting the hydraulic pump and motor to each other is regarded as the variation of load on the engine. Namely, this load controlling system controls the hydraulic servomechanism for controlling the displacement or displacements of the hydraulic pump or/and motor so as to decelerate the vehicle by an actuator driven by hydraulic pressure fluid led from the main fluid passage regarded as load on the engine.